DE-A 2 044 657 describes a process for preparing toluenediamine (TDA) by hydrogenation of dinitrotoluene (DNT) in the presence of nickel- or ruthenium-comprising hydrogenation catalysts.
EP-B1 1 138 665 describes a process for the catalytic hydrogenation of aromatic nitro compounds, in which the hydrogenation is carried out continuously using a catalyst comprising at least nickel and optionally aluminum. After carrying out the hydrogenation, the catalyst is separated from the reaction mixture in a separation zone.
The reaction mixture obtained when carrying out the hydrogenation of nitroaromatics in a reactor comprises not only the aromatic amines but also nitro and nitroso compounds which comprise, for example, the nitroaromatics used as starting materials or intermediates formed in the reactor. Nitro and nitroso compounds can decompose explosively, in particular on heating. For safety reasons, monitoring of the reaction mixture in respect of the concentration of nitro and nitroso compounds present therein is therefore important. The safety risk increases proportionately to the increase in the reactor size and to the reduction in the residence times in the reactor. It has to be ensured that these explosive compounds are reacted completely in the reactor before the reaction mixture is, for example, passed to a subsequent distillation.
A further problem encountered in the catalytic hydrogenation of nitroaromatics using nickel-comprising catalysts is that the catalysts are deactivated over time, with nitroaromatics accelerating the catalyst deactivation. The lower the activity of the catalyst, the smaller is the proportion of the starting materials converted into amines, so that the proportion of unreacted nitroaromatics remaining in the reactor increases and this in turn accelerates deactivation. Monitoring of the catalyst activity is therefore necessary, especially in order to introduce a sufficient amount of unexhausted catalyst into the reactor.
The monitoring of the concentration of nitroaromatics has hitherto been carried out either by UV/VIS methods after the catalyst has been separated off directly in the reaction mixture, in which case care has to be ensured that the detection limits achieved are significantly above the limit concentrations of nitroaromatics for catalyst deactivation, or by manual sampling, sample preparation and determination in the laboratory by polarographic or chromatographic methods. Disadvantages of manual sampling are the high personnel requirement and the risks arising from sampling. Owing to the time delay in the provision of analytical results, these manual methods are very disadvantageous for controlling the process.
In the case of such manual control, the aromatic amine/water mixture prepared can contain increased concentrations of nitroaromatics and these nitroaromatics can result in an appreciable irreversible reduction in the catalyst activity during relatively long periods of time. This can lead to complete cessation of the catalytic activity and thus to a stoppage of production.
These aromatic amines can be, for example, naphthalenediamines, xylylenediamines, toluenediamines, anilines and toluidines and also further aromatic amines and mixtures. The corresponding nitro compounds are dinitronaphthaienes, dinitroxylenes, dinitrotoluenes (DNT), aminonitrotoluenes (ANT), mononitrobenzene and mononitrotoluene.
The challenge faced by on-line determination of, for example, dinitrotoluene (DNT) in the matrix toluenediamine (TDA)/water with metallic catalyst is very great because of the high melting point of the sample of about 80° C. and the composition of the sample (many isomers), the solids content of metallic catalyst of from about 0.1 to 15% by weight and the need to measure DNT in the trace range of preferably from 3 to 30 ppm.
According to the prior art, the activity of the catalyst is followed by means of gas chromatography, for example by the method according to example 1 of WO 03/066571 A1.
WO 2006/089906 describes the determination of the concentration of nitro and nitroso compounds by means of UV/VIS spectroscopy. Here, separation of the catalyst from the reaction mixture is necessary. The detection limit of the determination is typically from 40 to 50 ppm.
The known methods are characterized by complicated handling and conditioning of samples, susceptibility to interference and a relatively high consumption of auxiliaries and utilities.
The continuous and fast monitoring and regulation of the actual concentration of nitroaromatics in the hydrogenation process is of particular importance for the preparation of aromatic amines.
The amines and diamines prepared are often processed further to produce isocyanates.